This invention relates to a vacuum gate valve which is employed as a sluice valve for a vacuum container in a semiconductor manufacturing device or the like, and more particularly to a seal structure thereof and to a guide mechanism for a stem shaft in the vacuum gate valve.
In a conventional vacuum gate valve of this type, in order to open and close its valve plate or to push the valve plate against the valve seat, a stem shaft coupled through a bellows to the valve plate is operated manually or with drive means such as a compressed air cylinder.
FIG. 7 is a sectional view showing the structure of an example of a conventional vacuum gate valve. In FIG. 7, reference numeral 1 designates a valve casing; 2, a valve plate which is reciprocated between two positions to open and close a vacuum path formed in the valve casing; 3, a stem shaft coupled to the valve plate 2; 9, a bonnet flange closing the valve casing 1; 70, a bushing fitted in a through hole formed in the bonnet flange 9 so as to slidably guide the stem shaft 3; 4, a bellows coupled between the bonnet flange 9 and a receiver on the stem shaft 3; and 10, an actuator for reciprocating the stem shaft 3.
In the above-described conventional vacuum gate valve, the stem shaft 3 and the bushing 70 are slidably engaged with each other in vacuum, and therefore they may, for instance, gall each other, thus making it impossible to open and close the valve. In addition, those components produce dust while wearing, which contaminates the pure vacuum atmosphere. Hence, the conventional vacuum gate valve cannot be employed for a semiconductor manufacturing device or the like.
In an another example of a conventional gate valve of this type, as disclosed by Japanese Patent Application (OPI) No. 312574/1988 (the term "OPI" as used herein means an "unexamined published application"), the valve plate is depressed with means such as a cam. Conventional gate valves having a mechanical structure can be disadvantageous in that components therein form abrasion dust when slid relative to one another.
In order to eliminate the above-described difficulty, Japanese Patent Application (OPI) No. 261573/1989 has disclosed a sluice valve in which a bellows is used to supply compressed air into a formed space to attain the sealing effect. This conventional device is still disadvantageous in that it is rather troublesome to switch the introduced air pressure in correspondence to the valve opening and closing operations, and if the bellows is broken, the compressed air flows into the vacuum container.
In order to solve the above-described problems, Japanese Patent Application (OPI) No. 218628/1989 has disclosed a sealing structure with a sealing surface and a closing member. However, this conventional device also involves problems. In particular, in the case when chambers on both sides of the closing member differ in pressure from each other, a great force acts on the closing member in the direction of axis of the opening, so that the piston rod may be bent, or the closing member may be shifted slightly in the axial direction, thus damaging the sealing member.